Heretofore, there is known a technology for projection-welding nuts and bolts to a steel sheet which has guide holes for the insertion of bolt shanks by setting the steel sheet on a lower electrode, lifting a positioning pin through a through hole in the lower electrode so as to project upwardly through a guide hole in the steel sheet, positioning a nut to be set on an upper surface of the steel sheet, thereafter lowering an upper electrode to press the nut against the steel sheet, and passing a current through the upper electrode and the lower electrode to weld the nut to the steel sheet. There are known techniques as disclosed in Japanese laid-open patent publications Nos. 7-9165 and 7-100660 for preventing a failure such as a reversed nut orientation or an improper nut attachment.
According to former Japanese laid-open patent publication No. 7-9165, a lower end of an actuating rod for lifting and lowering a guide pin (positioning pin) supports thereon contact pins which confront each other at a certain spaced interval, and a failure detecting circuit is formed between the actuating rod and the contact pins for detecting a nut setting failure or the like.
According to latter Japanese laid-open patent publication No. 7-100660, the position of the tip end of a detecting pin (positioning pin) which is coupled to the tip end of a coupling rod and is lifted and lowered by a cylinder is detected by a position detector for detecting a nut setting failure or the like.
In the case where the projected position of a positioning pin that can be moved into and out of the lower electrode is detected to detect a nut setting failure or the like, the device tends to be large in size due to the mounted detector, and if the actuating rod or the coupling rod which is coupled to the positioning pin is tilted when lifted or lowered, the positional accuracy is liable to suffer an error, making it difficult to detect a failure with accuracy. For example, a welding failure of a nut with a small projection, such as a hexagon nut, a round nut, a cap nut, a specially shaped weld nut, or the like cannot be detected. It is impossible to completely detect a failure of a square nut which can relatively easily be detected. Another problem is that the positioning pin tends to be worn because it is likely to frictionally engage in the through hole in the lower electrode.
If the gap between the coupling rod and a tube hole through which the coupling rod extends is reduced to prevent the coupling rod from being tilted when it is vertically moved, then when air is delivered into the tube hole for preventing sputtering particles, the air cannot flow smoothly, and fine sputtering particles are apt to enter the cylinder positioned therebelow through the tube hole. As a result, the cylinder tends to be worn and the stroke of the cylinder rod tends to be detected in error.
It is an object of the present invention to detect a projected position of a positioning pin with accuracy for thereby completely detecting a welding failure of a nut or the like and simultaneously to prevent sputtering particles produced when the nut or the like is welded from entering a lower cylinder through a tube hole of a holder, and also to make a welding device compact.